Implement single-pass allocation-free views for calculating num_items().

[jmacd]

Pre-filing checklist

• I searched existing issues and didn't find a duplicate

Component(s)

Rust OTAP dataflow (rust/otap-dataflow/)

Bug Description

We should not need to allocate memory to count the number of items in an OTLP payload.

Steps to Reproduce

n/a

Expected Behavior

This is true because we only interpret a single tag at each level of the hierarchy, never need to save a field we skip.

Actual Behavior

One allocation per num_items().

OTel-Arrow Version

HEAD

Environment

No response

Configuration

Log Output

Additional Context

No response